1. Field of the Invention
The present invention relates to a ceramic high-temperature superconductor and its use in heavy-current electrical engineering.
The invention also relates to the further development of inductive current limiters for alternating current, bodies electromagnetically coupled to the operating current and with greatly variable resistance behavior (superconductors) being used.
In particular, it relates to a device for inductive current limiting of an alternating current making use of the superconductivity of a ceramic high-temperature superconductor, there being provided an induction coil which consists of at least one winding and through which current flows, as well as a body made of a super-conducting substance arranged concentrically to the latter.
2. Discussion of Background
Current limiters in the high-current range of above 1 kA alternating current can be used advantageously in distribution networks, as generator protection, transformer protection or apparatus protection. All these devices are ones which counter the current from a particular value onwards (for example in the case of five-fold mains current) gradually to suddenly with a much higher resistance and limit it in this manner to a maximum possible value. Devices have been proposed for this purpose based on the principle of superconductivity, various embodiments being known:
Resistive (ohmic) limiter: In the case of a short-circuit, a superconducting element becomes normally conductive and commutes the current to a limiting resistance (cf. K. E. Gray and D. E. Fowler, "A superconducting fault-current limiter", J. Appl. Phys. 49, pages 2546-2550, Apr. 1978). PA1 Inductive limiter: Transformer with direct current biasing. A transformer has a superconducting secondary winding carrying a direct current which magnetizes the iron core until saturation. The impedance of the apparatus is thus kept low. In the case of a short-circuit, the core is removed from the saturation and the impedance rises greatly (cf. B. P. Raju and T. C. Bartram, "Fault-current limiter with superconducting DC bias", IEEE Proc. 129, Pt. C. No. 4, pages 166-171, Jul. 1982). PA1 Inductive limiter: Transformer with short-circuited superconducting secondary winding. The short-circuited secondary winding considerably reduces the inductance of the transformer in normal operation. In the case of a short-circuit, the superconductor is switched to the normally conducting state and the inductance rises greatly: resistive limiter with transformer with impedance matching (cf. U.S. Pat. No. 4,700,257). PA1 As a result of the jacketing of the core over the full winding length of the induction coil, a complete shielding of the core from the magnetic field is achieved in normal operation. This leads to a substantial reduction of losses in normal operation, in particular iron losses and stray losses are greatly reduced or avoided completely. PA1 As a result of the arrangement of the superconducting jacket in the field area of the current-carrying winding, quenching is greatly supported in the case of a short-circuit. It is known that often only an unsatisfactory quenching can be achieved by means of induced currents or current pulses. The magnetic field of the winding which is always automatically present ensures a smooth transition of the superconductor to the normal state. PA1 The arrangement of the superconducting jacket in the field area of the current-carrying winding leads to a distinctly higher electrical resistance of the superconducting layer in the normal conducting state. This is a consequence of the great magnetic field dependency of the critical currents as well as of the current/voltage characteristics found in ceramic oxide superconductors. The high electrical resistance of the jacket in the normal conducting state increases the choke effect of the coil, which would otherwise be reduced by induced eddy currents in the jacket.
The known alternating current limiters are bulky and complex and require, since they are based on classic superconductors, considerable cooling outlay. There is therefore a requirement for the further development and refinement of current limiters.